In holographic recording, an interference pattern is generated on a storage medium, using an information beam which has two-dimensional page information and a reference beam which is separated from the same beam flux from which the information beam is separated, and the interference pattern is recorded. When reproducing the recorded information, the reference beam used for the recording is applied onto the recorded area in the storage medium, and a reproduction beam which comes from the storage medium is brought to focus on a two-dimensional-page-beam detector to form an image.
Advantages in holographic recording includes multiplex recording, i.e. recording a plurality of pieces of information on the same area in a storage medium. For example, Patent Document 1 discloses a shift-multiplex holographic recorder. FIG. 11 shows a typical configuration of such a holographic recorder.
A holographic recorder X in FIG. 11 is an apparatus designed to record information on a storage medium Y as well as reproduce the recorded information by a shift-multiplex method. The storage medium Y includes a recording layer 991, which is made of a recording material such as photopolymer, sandwiched between a substrate 990 which is made of a glass plate for example, and a reflection layer 992. The storage medium Y is capable of recording interference patterns made by the information beam and the reference beam, in the form of refraction index changes of the recording material. Also, by applying only the reference beam to the recorded interference pattern, optical information which represents the recorded information is extracted.
In the holographic recorder X, a laser beam from a light source 901 is made into a parallel beam by a collimator lens 902, and then introduced to a beam splitter 903. The laser beam is then split by a beam splitter 903 into an information beam which comes out in a downward direction as in FIG. 11, and a reference beam which comes out in a rightward direction in the figure. The information beam is expanded in its beam diameter by combined lenses, i.e. beam expanders 904a, 904b, and then introduced to a spatial light modulator 905. The information beam is modulated by the spatial light modulator 905 into a beam which includes two-dimensional page information. The information beam which has left the spatial light modulator 905 passes through a beam splitter 906 and then through an object lens 908. The information beam is then condensed by the object lens 908 onto a recording layer 991. On the other hand, the reference beam is reflected by reflection mirrors 910a, 910b, and applied to the portion on the recording layer 991 where the information beam is applied.
Information recorded on the recording layer 991 by the information beam and the reference beam as described is reproduced when only the reference beam is applied to the recording layer. Specifically, when the reference beam is applied to the recording layer 991, a reproduction beam is generated which includes the two-dimensional page information. The reproduction beam is turned back by the reflection layer 992 in an upward direction as in FIG. 11, and enters the object lens 908. The reproduction beam, which is made into a parallel beam by the object lens 908, is then turned to the left as in FIG. 11 by the beam splitter 906, and enters a photo detector 915. The photo detector 915 extracts the two-dimensional page information contained in the reproduction beam.
The holographic recorder X uses shift-multiplex method in which multiplex recording of information is performed while making a very fine shift on the recording area. In the shift-multiplex method, accurate application of the reference beam to the storage medium Y is essential in order to make recording as well as reproducing, and for this reason, a complicated system is necessary as a means for controlling the storage medium Y. There is a similar problem in holographic recorders which use angle-multiplex method in which multiplex recording is accomplished by varying an incoming angle of the reference beam. In this case, it is difficult to control a reflection mirror which is used to apply the reference beam.
Patent Document 1: JP-A-2005-235312